Such a method is known from WO 01/095550 A.
Furthermore, the invention relates to a star coupler for a communication system for the transmission of Ethernet messages in a distributed real-time system including a plurality of network node computers, e.g. four network node computers, each of which comprises at least one communication controller, the communication system comprising one or more communication channels via which the network node computers are connected to each other and one or more intelligent star couplers being disposed in each communication channel.
In the text to follow, reference is made to the literature indicated below:
[1] U.S. Pat. No. 5,694,542
[2] EP 0 658 257
[3] U.S. Pat. No. 5,887,143
[4] AT 407 582
[5] AT 408 383
[6] Austrian Patent Application 1723/2001
[7] Austrian Patent Application 429/2001
[8] IEEE Ethernet Standard 802.3 at URL: HTTP://standards.ieee.org
[9] Kopetz, H. (1997). Real-Time Systems, Design Principles for Distributed Embedded Applications; ISBN: 0-7923-9894-7. Boston. Kluwer Academic Publishers
[10] Sharon, O., Spratt, M., “A CSMA/CD compatible MAC for real-time transmission based on varying collision intervals”. In: INFCOM'98. Seventh Annual Meeting Joint Conference of the IEEE Computer and Communications Societies. Proceedings. IEEE, Volume: 3, 1998, pp. 1265-1272 vol. 3.
In the past twenty years, the IEEE Ethernet Standard 802.3 [8] has gained such wide acceptance that, because of the mass market for Ethernet controllers in the personal computer area, the costs for Ethernet-based communication systems have dropped very sharply. For these cost reasons, Ethernet is also used increasingly in real-time data processing, although the existing Ethernet protocol possesses no good real-time properties, such as minimal jitter.
A CSMS/CD system is known from [10] in which messages are subdivided into such low and high priority, the message with high priority being given the preference when there is a conflict between two messages.
However, the real-time properties of the Ethernet protocol cannot be substantially improved with the procedure proposed here by itself.
One object of the invention is to enable the transmission of Ethernet messages with good real-time properties.
This objective is achieved using a method of the type mentioned at the outset, in that according to the invention, when there is a time conflict between ET and TT messages, the transport of the ET message that is in conflict is aborted in order to be able to transport the TT message with the constant delay time.
Furthermore, the objective mentioned at the outset is achieved using a star coupler mentioned at the outset that is set up according to the invention to distinguish between conventional Ethernet messages (ET messages) and time-triggered Ethernet messages (TT messages), to transport the TT messages having an a priori known constant delay time between transmitter and receiver, whereby, when there is a time conflict between ET and TT messages, the transport of the ET message that is in conflict is aborted in order to be able to transport the TT message with the constant delay time.
In contrast to the “non pre-emptive” solution disclosed in [10], in the present invention it is not necessary to wait for the end of the transmission of a message having low priority; rather, the low priority message is aborted in order to be able to transmit the high-priority message (“pre-emptive”). As a result it is also not necessary to wait on the maximum run time of messages having low priority, and the constant latency can thus also be kept short.
Also in WO 01/095550 A, such a solution according to the invention is not shown; there, when there is a time conflict between ET and TT messages, the transport of the ET message in conflict is merely delayed.
By guaranteeing a constant delay time, it is possible to achieve a high control engineering precision. The constant delay time is therefore of special significance for the reason that, as is known from the theory of clock synchronization, the variability of the delay time (this is the difference between the maximum and minimum delay times) makes the precision of the clock synchronization worse. An a priori known constant delay time can be considered in the clock synchronization algorithm and therefore has no effect on the precision of the clock synchronization. An imprecise clock synchronization brings about a poor time basis because the granularity of the global time must be greater than the precision of the clock synchronization. A coarse granularity of the clocks brings about an imprecise time resolution of events. Furthermore, the variability of the delay time also determines the precision of the synchronization of distributed actions in a distributed computer system.
The present invention makes it possible to improve substantially the real-time properties of a communication system. This new communication system supports the parallel operation of event-triggered and time-triggered Ethernet messages in a single communication system. In the following, the classical Ethernet messages are characterized as ET (event-triggered) messages and the time-triggered Ethernet messages as TT (time-triggered) messages. The TT messages have a constant delay time and minimal jitter.
The following significant economic advantages are produced by the invention. The minimal jitter of the TT-messages enables the development of closed-loop control circuits of high control engineering quality. The TT messages enable the development of a global time with good precision. Global time supports the generation of precise local time stamps in the data acquisition and makes it possible to improve the temporal specification of the interfaces. Moreover, conventional Ethernet controllers may be used without modification.
The method of the invention can be produced especially simply if the constant delay time is selected in such a manner that the output channels of the star coupler may be cleared for the transport of the TT message to be received.
In one embodiment, there is an indication in a designated field of the message of whether the message is a TT message or is an ET message.
Furthermore, an optional time field that indicates the transmission instant of the message may be contained within a TT message.
In this context it is advantageous if it is already established by a priori planning that a time interval of at least the constant delay time is observed between the transport of two TT messages.
Furthermore, the aforementioned objective is achieved using a system mentioned at the outset that is set up according to the invention to distinguish between conventional Ethernet messages Ethernet (ET messages) and time-triggered Ethernet messages (TT messages) and to transport the TT messages with an a priori known constant delay time between transmitter and receiver, whereby, when there is a time conflict between ET and TT messages, the transport of the ET message that is in conflict is aborted in order to be able to transport the TT message with the constant delay time.
As already mentioned above, it is advantageous if the constant delay time is selected in such a manner that the output channels of the star coupler can be cleared within this delay time for the transport of the incoming TT message.
Moreover, it may be provided that there is an indication in a designated field of the message of whether the message is a TT message or an ET message.
Furthermore, an optional time field that indicates the transmission time of the message may be contained within a TT message. In this context it is advantageous if it is already established by a priori planning that a time interval of at least the constant delay time is observed between the transport of two TT messages.
In a concrete embodiment of the communication systems, a configuration message is provided a priori to the star coupler specifying times during which messages received by the star coupler are TT messages.
In this context, the star coupler distinguishes between TT messages and ET messages and transports the TT messages with an a priori known constant delay time, and, when there is a time conflict between ET and TT messages, it aborts the transport or the ET message that is in conflict in order to be able to transport the TT message with the constant delay time.
Provision is then made that after the on-time transmission of a TT message the star coupler re-transmits the ET message that was in conflict and was aborted. Furthermore, it may be provided that the star coupler synchronizes its local clock using the time field contained within a TT message.
In this context, it is especially beneficial if the star coupler in a fault-tolerant manner synchronizes its local clock using the time fields contained within a plurality of TT messages. In addition, it may be provided that the star coupler is connected to the replicated star couplers within a cluster of network node computers via a dedicated unidirectional channel on which all TT messages that the star coupler transports are output.
In addition, it is also still possible that the star coupler, on the basis of its local time, checks whether the TT message arrives within an a priori known time window around the instant of transmission, and the star coupler, when it receives a TT message early or late, garbles the message in such a manner that all correct receivers detect the message as faulty. The star coupler decodes each TT message and re-codes it on the basis of its local timing module.
The star coupler reads one or more selected fields of TT messages and checks during the delay time whether the content of these fields corresponds to known criteria that were communicated a priori to the star coupler via a configuration message. If it does not correspond, the message is garbled in such a manner that all correct receivers detect the message as faulty. Moreover, provision is also made according to the invention that the communication controller synchronizes its local clock using the time field contained within a TT message.
The communication controller in a fault-tolerant manner synchronizes its local clock using the time fields contained within a plurality of TT messages. Moreover, the communication controller autonomously transmits a TT message accepted by an application executed on a network node computer as soon as the transmission instant indicated in the time field in the message is reached.
Moreover, provision is made that the communication controller distinguishes between ET and TT messages, and the communication controller provides the ET messages to the local application software corresponding to the event semantics, a new message being placed in a waiting queue from which it is read in a consumptive manner by the application software, and the communication controller provides TT messages to the local application software corresponding to the status semantics, a new message replacing the old one and the reading by the local application software occurring in a non-consumptive manner.
Finally, the communication controller has two or more communication channels on which identical copies of a TT message are provided, and the one communication operation is considered successful if a valid TT message is received on time on at least one of these redundant channels.